Executable Scripts with Java

Running Java from Source

Since Java 11 the java command does have the --source option, which allows running Java code which is given in source. For demonstration purposes I’m using a Java 13 installation, but the sample will work with all Java versions >11.

$ java -version
openjdk version "13.0.1" 2019-10-15
OpenJDK Runtime Environment (build 13.0.1+9)
OpenJDK 64-Bit Server VM (build 13.0.1+9, mixed mode, sharing)

When the --source feature is combined with the Shebang of modern shells like so:

$ cat hellojava.sh
#!/home/gunther/.jenv/shims/java --source 11
public class App {
    public static void main(String ...args){
        System.out.println("Hello, Java!");
    }
}

the file - given Posix file attributes include execute permission - can be executed like a shell script:

$ ./hellojava.sh
Hello, Java!

Java is still a compiled language, ie. before the script starts, the Java compiler still need to generate the byte-code to be executed. This impacts the startup time of large scripts - Java applications are in general not known for starting quickly. Nevertheless, the --source option allows more use cases implemented with Java.

jenv

The one or the other may be wondering about the java command of the Shebang definition. It actually points to the jenv tool, which eases management of multiple Java installations in parallel. Generally, the Shebang command should the absolute path printed out by the which command:

$ which java
/home/gunther/.jenv/shims/java

$ jenv version
openjdk64-13.0.1 (set by /home/gunther/.jenv/version)

If you’re looking for a more portable Shebang definition, you could apply the env command as follows:

#!/usr/bin/env -S java --source 11
...

This definition executes the Java installation (first) found in the PATH, ie. the default Java installation on the system, regardless the installation directory.

Before Java Version 11

It becomes less relevant with every day, but there are still organizations supporting legacy systems that run on Java 8. In such a environment the --source option is not available. Nevertheless, there’s still a way to package Java applications into an executable file.

Let’s switch to Java 8 (thanks to jenv this is a piece of cake :-):

$ java -version
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_222-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.222-b10, mixed mode)

The Java application is the same as before, just placed into a separate file which follows the Java naming convention, ie. file name equals class name.

$ cat HelloJava.java

public class HelloJava {

	public static void main(String[] args) {
		System.out.println("Hello, Java!");
	}
}
$ javac -version
javac 1.8.0_222

$ javac HelloJava.java

Note, that for sake of simplicity the Java source and class files are in the current working directory, ie. the class belongs to the default package. But the demonstrated approach will work for any Java package structure.

In the next step the compiled class is packaged into a JAR file:

$ cat manifest.txt
Main-Class: HelloJava
$ jar cvfm hellojava.jar manifest.txt HelloJava.class
added manifest
adding: HelloJava.class(in = 424) (out= 287)(deflated 32%)

Note, that the JAR’s manifest defines the HelloJava class as entry point for execution.

Packaged the application as described, the executable can be created by the following commands:

$ echo '#!/home/gunther/.jenv/shims/java -jar' >hellojava8.sh
$ cat hellojava.jar >>hellojava8.sh
$ chmod +x hellojava8.sh
$ ./hellojava8.sh
Hello, Java!
$

Instead of placing the source code into the body of the shell script, the JAR file itself is appended to the Shebang line, on which the --source is replaced by the -jar option.

Though, the steps to create the executable Java application can be automated, the scripting experience gets partly lost by the need to explicitly compile and package the Java classes. On the other hand, the startup time is better compared to the source approach.

Executable Scripts with Java

Running Java from Source

Before Java Version 11

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